1. Field of the Invention
The present invention relates to a method for producing a transparent alumina sintered body.
2. Description of the Related Art
Highly dense, highly pure polycrystalline alumina is known to be optical translucent and can be used, for example, in high-pressure sodium lamp arc tubes, highly heat-resistant window materials, members of semiconductor devices, and substrates for optical components. Unfortunately, the crystal structure of alumina is optically anisotropic. If alumina grains in an alumina sintered body are randomly oriented, the crystal grains have different refractive indices. Thus, the sintered body scatters light and the transmittance thereof decreases. It is desirable that alumina grains in a transparent alumina having a high in-line transmittance be uniaxially oriented. For example, NPL 1 discloses a method for producing transparent alumina. In this method, highly pure, highly aligned, and highly dense transparent alumina is produced by using a high magnetic field.
Also, there is known a method in which in order to improve the sinterability of the alumina, a fluoride is added to an alumina raw material powder before sintering in a process for producing an alumina sintered body. For example, in PTLs 1 and 2, a dense alumina sintered body is produced by mixing an alumina powder and a fluoride powder, and then sintering the mixture at a low temperature of 1300° C. or less. In PTL 3, a corrosion-resistant, impact-resistant, and durable alumina sintered body is produced by adding a fluoride to an alumina raw material powder, pulverizing and mixing the materials to an average particle diameter of 2 μm or less, compacting the mixture, and sintering the compact in air at a temperature of 1600° C. to 1800° C.